To lock two telescopic elements in a given longitudinal position it is known practice for the end of the outer element to be provided with a conical ring with longitudinal slots and with a nut so as to squeeze this split ring and press it against the inner element.
Patent FR-A-2 768 200 also discloses how to immobilize two telescopic elongate elements in one direction by cutting the end of the outer element at an angle and arranging around the inner element a washer which, when it bears against the end of the chamfer, adopts an inclination and locks the inner element in the direction of pushing it into the outer element.